Lubricant viscosity is an important element for equipment builders and automotive manufacturers to consider. The viscosity of the lubricant is directly related to the thickness of the protective lubricant film formed in service. The viscosity of the lubricant also affects its circulation rate in small passageways in the equipment being lubricated. Equipment components are therefore specifically selected and designed to be used with lubricants of a specified viscosity. Maintenance of suitable viscosity is therefore critically important for proper operation of lubricated equipment.
Resistance to degradation is desirable for lubricants in service. Lubricants decompose via a number of different mechanisms or pathways: thermal, oxidative and hydrolytic mechanisms are well known. During thermal and hydrolytic decomposition, the lubricant is usually broken down into smaller fragments. During oxidative decomposition, higher molecular weight sludges are often formed. In each of these pathways, byproducts are also formed, often acids. These byproducts can catalyze further degradation, resulting in an ever increasing rate of degradation.
Since the lubricant viscosity is affected by the various decomposition pathways, and maintenance of lubricant viscosity is critical, lubricant viscosity is frequently checked in almost all lubricant applications. The in-service viscosity is compared against the fresh oil viscosity to detect deviation indicative of degradation. Viscosity increase and viscosity decrease are both signs of potential lubricant degradation.
In industrial lubricant application, lube viscosity is classified by ISO viscosity grade. ISO Viscosity Grade standards have a ±10% window centered around the specified viscosity. For example, lubricants with a viscosity of 198 cSt and 242 cSt would be considered just in-grade for the ISO VG 220 specification. Lubricants which fall out of the ISO VG specifications may still be effective lubricants in service. However, since known degradation mechanisms result in viscosity changes, many equipment owners will replace lubricants which fall outside of the ISO VG limits. This decision may also be driven by such factors as equipment warranty or insurance requirements. Such considerations may be very important for expensive industrial equipment. The cost of downtime for lubricant related failures can also play a role in the lubricant change-out decision. If downtime is very expensive relative to the cost of lubricant, tighter criteria for lubricant change-out will often be used.
Other lubricants, such as automotive engine lubricants or transmission fluids or automotive gear oil or axle lubricants or grease, are also classified by different viscosity ranges, as described by SAE (Society of Automotive Engineers) J300 or J306 specifications, or by AGMA (American Gear Manufacturers Association) specifications. These lubricants will have the same issues as industrial lubricants described in previous paragraph.
One benefit of premium lubricants is the potential for extended life, reducing the change-out interval. Extended lubricant life is one feature that offsets the higher initial fill cost for premium lubricants. In order to achieve an extended lubricant life, premium lubricants must demonstrate a more stable viscosity in service. Using higher quality base stocks and advanced additive systems, these lubricants counter the effects of thermal, oxidative and hydrolytic attack.
In addition to the chemical mechanisms for viscosity change discussed above, however, another mechanism for viscosity change is mechanical in nature. Viscosity loss due to severe shear stress in a lubricant occurs when lubricant molecules are fractured in high shear zones in the equipment. These zones exist in many loaded gears, roller bearings, or engine pistons at high rpm. As lubricant is circulated through these zones, different parts of the lubricant base stock molecules are subjected to different mechanical stress, causing the molecules to permanently break down into smaller pieces, resulting in reduction in lubricant viscosity. This shear viscosity breakdown is specifically problematic with high viscosity lubricant base stocks due to their high molecular weight components.
A sheared-down lubricant may still retain excellent resistance to thermal, oxidative or hydrolytic degradation, however, a lubricant with out of range viscosity can lead to premature change-out for the same reasons cited earlier, e.g., due to warranty, insurance or downtime prevention. On the other hand, a sheared-down lubricant may initiate other undesirable degradation processes, such as oxidation, hydrolysis, etc., leading to reduced lubricant life time. Thus it is desirable to avoid the loss of viscosity by mechanical mechanism as well as chemical mechanisms discussed above.
One important example of the need for extended resistance to mechanical breakdown is in wind turbines gear boxes. Generally, these operate with slow moving gears which are highly loaded, and thus highly susceptible to mechanical breakdown. In addition, wind turbines tend to be located in inaccessible locations, such as the North Sea. The susceptibility to mechanical breakdown and inaccessibility is also present in many hydraulic applications. Accordingly, it would be highly desirable to have a lubricant for wind turbine gear boxes, hydraulic applications, and the like, wherein the lubricant has a high resistance to mechanical breakdown over a very long period of time.
The viscosity loss by mechanical shear down of a lubricant or lubricant base stock can be measured by several methods, including Tapered Roller Bearing (TRB) test according to CEC L-45-T-93 procedure, Orbahn (ASTM D3945) or Sonic Shear Tests (ASTM D2603). The TRB test is believed to correlate better to the actual field shear stability performance of viscous fluids than the other shear tests.
One important variable in determining susceptibility of a base stock to shear viscosity breakdown is its molecular weight distribution (MWD). Molecular weight distribution (MWD), defined as the ratio of weight-averaged MW to number-averaged MW (=Mw/Mn), can be determined by gel permeation chromatography (GPC) using polymers with known molecular weights as calibration standards. Typically, base stocks with broader MWD are more prone to shear viscosity breakdown than base stocks with narrower MWD. This is because the broad MWD base stock usually has a larger high molecular weight fraction, which breaks down easier in high stress zones than the narrow MWD base stock, which has a much lower high molecular weight fraction.
To obtain shear stable lubricants, it is therefore desirable to have a narrow MWD. One way to achieving narrow MWD is to use metallocene catalysts, which was discovered by Sinn and Kaminsky based on early transition metals (Zr, Ti, Hf) with methylaluminoxane (MAO). Soon after the appearance of metallocene catalysts in 1980 their advantages over the conventional multi-site Ziegler-Natta and chromium catalysts were recognized. Thus, they are highly active catalysts exhibiting an exceptional ability to polymerize olefin monomers, producing uniform polymers and copolymers of narrow molecular weight distribution (MWD of less than or equal to about 2) and narrow chemical compositional distribution, controlling at same time the resulting polymer chain architectures.
WO1999067437 disclosed the use of MAO-activated metallocenes with Al/Metallocene mole ratios of 100:1 to 10,000:1 to give poly-1-decene with excellent shear stability when compared to polymethacrylates in multi-grade transmission oil formulation. Products described therein were made in a batch mode and no distinction was made of using other processing methods to improve the molecular weight distribution and the shear stability of the fluid.
The use of single-site metallocene catalysts in the oligomerization of various alphaolefin feeds is known per se, such as in WO2007/011832, WO2007/011459, WO2007/01 1973, and PCT/US2007/010215.
A lubricant having higher stability to the various degradation mechanisms discussed above than is currently available, and able to meet, by itself or in a blend, industrial specifications such as ISO VG and SAE grades is highly sought after.
The present inventors have surprisingly discovered that high shear stable lubricant base stocks may be prepared by contacting alphaolefin feedstocks with single-site metallocene catalysts in a mixed flow or continuous stirred tank reactors.